1. Field of the Invention
The present invention relates to a manufacturing method of a honeycomb structure. More particularly, it relates to a manufacturing method of a honeycomb structure that is superior in formability, has a high manufacturing efficiency, and can assure a stable porosity with respect to an obtained honeycomb structure and maintain a required strength.
2. Description of the Related Art
A honeycomb structure formed of cordierite having a low thermal expansion coefficient is used in order to adsorb/absorb, e.g., NOx, CO, or HC contained in an exhaust gas from an automobile by, e.g., a carried catalyst. As such a honeycomb structure, one having pores (air holes) formed in a partition wall is usually utilized to carry, e.g., a catalyst. As a method of forming the pores, there is disclosed a method of blending a pore forming agent consisting of solid particles or hollow particles in a forming raw material and burning off the pore forming agent to form the pores when firing a formed body (see, e.g., Patent Documents 1 to 3). Further, a method using a hygroscopic resin as the pore forming agent is disclosed (see, e.g., Patent Documents 4 and 5). Furthermore, a method of manufacturing a honeycomb structure having pores in a partition wall without using a pore forming agent is disclosed (see, e.g., Patent Documents 6 and 7).
[Patent Document 1] Japanese Patent No. 2938740
[Patent Document 2] JP-A-2003-40687
[Patent Document 3] JP-A-2004-315346
[Patent Document 4] A brochure of International Publication No. WO2005/63360
[Patent Document 5] JP-A-11-71188
[Patent Document 6] JP-B-7-61892
[Patent Document 7] JP-A-63-40777
Each of the inventions disclosed in Patent Documents 1 to 3 is a manufacturing method of a porous ceramics structure that obtains a honeycomb formed body by extrusion forming and then fires this formed body to acquire a honeycomb structure, and it uses solid-core or hollow flammable particles as a pore forming agent and forms air holes when the particles are burnt off at the time of firing. However, when the solid particles are used as the pore forming agent, since each of the particles has a solid core, there can be obtained an advantage that these particles are hard to be crushed at the time of mixing/kneading a forming raw material and a stable porosity can be obtained. However, there is a problem that a die for extrusion forming is clogged with these particles and a defect, e.g., a crack in a partition wall occurs in the formed body. Furthermore, there is also a problem that an extrusion pressure is increased and the die for extrusion forming is deformed. Moreover, since these particles have a high calorific value at the time of firing, there is a problem that a failure, e.g., a crack due to a thermal stress or an internal defect often occurs. On the other hand, when hollow particles are used as the pore forming agent, since the particles are hollow and have a low calorific value at the time of firing, occurrence of the above-explained defect can be suppressed. However, since the particles are apt to be crushed at the time of mixing/kneading/forming a raw material, there arises a problem that a stable porosity cannot be assured and filter characteristics are degraded.
The invention disclosed in Patent Document 4 is a manufacturing method of a honeycomb structure that subjects a forming raw material containing a hygroscopic resin blended therein as a pore forming agent to extrusion forming to obtain a honeycomb formed body and then fires this formed body to acquire a honeycomb structure. This method can reduce a pressure required at the time of forming in particular and suppress a defect or deformation, and is superior in a yield ratio. However, it has a problem that a honeycomb structure having a porosity of 40% or above alone can be obtained. In particular, a thin-walled honeycomb used for emission gas purification in an automobile has a problem that a strength is reduced when the porosity is 40% or above.
The invention according to Patent Document 5 obtains a pellet type formed body based on extrusion forming, then granulates this formed body to acquire a spherical formed body, and dries and fires this spherical formed body to obtain a porous body. Since this invention performs granulation after extrusion forming, it has an advantage that product characteristics do not vary depending on a defect at the time of extrusion forming (formability at the time of extrusion forming). The invention disclosed in Patent Document 5 does not concern the manufacturing method of a honeycomb structure. However, in a case where this manufacturing method is applied to a honeycomb formed body, this invention has not only a problem that a drying time is prolonged when a water absorption magnification of a hygroscopic resin exceeds 100 magnifications since a formed body formed into a honeycombs structure contains a large amount of water but also a problem that a large amount of power for drying is required to increase a cost for drying. Additionally, this invention also has a problem that deformation readily occurs and a yield ratio is apt to be lowered since a hardness degree of the formed body having the honeycomb structure is reduced or a crack at the time of drying is increased. Further, this invention also has a problem that a stable porosity cannot be assured and filter characteristics are degraded since a gel strength is lowered and a hygroscopic resin is apt to be crushed during forming.
The inventions disclosed in Patent Document 6 and 7 are methods of manufacturing a porous honeycomb structure without using a pore forming agent. These methods have a problem that fluidity of a forming raw material in extrusion forming is low and formability is poor as compared with the manufacturing method using a hygroscopic resin as the pore forming agent. Further, these methods do not have a conception of using the hygroscopic resin as a forming auxiliary agent for the purpose of improving formability. Therefore, a grain size of a cordierite forming raw material required to maintain a strength necessary as a honeycomb structure while using the hygroscopic resin is not examined at all in these method.